A division in Saturn's ring in the outer part of the C ring. It is about 87500 km from Saturn's
center and is 500 km wide. The gap was discovered in 1980 by Voyager 1.

Not discovered by J. C. Maxwell, but named in his honor; → maxwell;
→ gap.

Maxwell's demon

پری ِ ماکسول

pari-ye Maxwell

Fr.: démon de Maxwell

A → thought experiment meant to raise questions
about the possibility of violating the
→ second law of thermodynamics.
A wall separates two compartments filled with gas. A little "demon" sits by a tiny
trap door in the wall. He is able to sort hot (faster) molecules from cold
molecules without expending energy, thus bringing about a general
decrease in → entropy and violating the second law of thermodynamics.
The → paradox is explained by the fact that such a demon
would still need to use energy to observe and sort the molecules.
Thus the total entropy of the system still increases.

Named after James Clerk Maxwell (→ maxwell), who first thought of
this experiment; → demon.

→ maxwell. It should be emphasized
that the equations originally published by James Clerk Maxwell in 1873 (in
A Treatise on Electricity and Magnetism)
were 20 in number, had 20 variables, and were in scalar form. The
German physicist Heinrich Rudolf Hertz (1857-1894) reduced them to 12 scalar
equations (1884). It was the English mathematician/physicist Oliver Heaviside
(1850-1925) who expressed Maxwell's equations in vector
form using the notations of → gradient,
→ divergence, and → curl
of a vector, thus simplifying them to the present 4 equations (1886).
Before Einstein these equations were known as Maxwell-Heaviside-Hertz equations,
Einstein (1940) popularized the name "Maxwell's Equations;"
→ equation.

Maxwell's rule

رزن ِ ماکسول

razan-e Maxwell

Fr.: règle de Maxwell

Every part of a deformable electric circuit tends to move in such a direction as to
enclose the maximum magnetic flux.

The incompatibility between → Galilean relativity
and Mawxell's theory of → electromagnetism.
Maxwell demonstrated that
electrical and magnetic fields propagate as waves in space.
The propagation speed of these waves in a vacuum is
given by the expression c = (ε0.μ0)-0.5,
where ε0 is the electric → permittivity
and μ0 is the magnetic → permeability, both
→ physical constants.
Maxwell noticed that this value corresponds exactly to the
→ speed of light
in vacuum. This implies, however, that the speed of light must also be a universal
constant, just as are the electrical and the magnetic field constants! The
problem is that → Maxwell's equations do not relate
this velocity to an absolute background
and specify no → reference frame
against which it is measured. If we accept that the principle of relativity
not only applies to mechanics, then it must also be true that
Maxwell's equations apply in any → inertial frame,
with the same values for the universal constants. Therefore, the speed of light
should be independent of the movement of its source. This, however, contradicts the
vector addition of velocities, which is a verified principle within
→ Newtonian mechanics.
Einstein was bold enough to conclude that the principle of Newtonian
relativity and Maxwell's theory of electromagnetism are incompatible!
In other words, the → Galilean transformation
and the → Newtonian relativity
principle based on this transformation were wrong. There exists, therefore,
a new relativity principle, → Einsteinian relativity,
for both mechanics and electrodynamics that is based on the
→ Lorentz transformation.

1) A well used in Antiquity from bottom of which the sky could be observed
during the day with a better contrast. The aperture of the well reduced the
light diffused by the sky.
2) A vertical tunnel in → Paris Observatory
(built in 1667), from the top
roof to the underground vaults, 55m deep. This well had the purpose of
observing the stars near the → zenith
and measure their → parallaxes resulting from
the Earth motion around Sun. A long plumb line was attached to the top
of the well. Astronomers thought they could measure the stellar shifts
with respect to the plumb line.
The problem was, however, the lack of stability of the images, because the
well acted in fact as a chimney generating turbulence.
So that the zenithal well was hardly used.
See also: → zenith telescope.